This invention relates generally to the measurement of underwater acoustic intensity, and more specifically to a novel apparatus having advantages over known underwater acoustic intensity probes.
Underwater acoustic intensity has been measured by various devices. For example, U.S. Pat. No. 4,982,375, dated Jan. 1, 1991 describes an acoustic intensity probe using multiple hydrophones and a fast Fourier transform analyzer. U.S. Pat. Nos. 3,274,539, dated Sep. 20, 1966 and 3,311,873, dated Mar. 28, 1967, describe devices which use a hydrophone and an accelerometer in combination. U.S. Pat. No. 5,392,258, dated Feb. 21, 1995, describes a device using a hydrophone and a geophone in combination. A similar device is described in T. B. Gabrielson et al., "A Simple Neutrally Buoyant Sensor for Direct Measurement of Particle Velocity and Intensity in Water," J. Acoust. Soc. Am. 97, 2227-2237 (1994). Each of these devices uses at least one piezoelectric sensor, either in a hydrophone, or in an accelerometer.
A piezoelectric sensor is inherently a high impedance device. Because of its high impedance, the sensor, by itself, cannot deliver an output signal over a long distance without significant attenuation. Accordingly, it is usually necessary to locate a preamplifier in close proximity to the sensor, and to provide impedance matching networks at the input and output of the preamplifier, at the location of the sensor.
Another drawback of the intensity probe using hydrophone pressure sensors is that the pressure sensors are directly exposed to flow as the probe moves through a body of water (or as the body of water flows past the probe), and consequently subject to noise generation due to turbulent flow.